1. Field of the Invention
The invention generally relates to the field of communications. More specifically the invention is related to interference suppression for use in coded signal communications, such as Code Division Multiple Access (“CDMA”) communications, Wideband CDMA (“WCDMA”) communications, Global Positioning System (“GPS”) communications, Broadband CDMA communications, Universal Mobile Telephone Service (“UMTS”) communications and combinations thereof.
2. Discussion of the Related Art
In CDMA telephony, coded signals are used to communicate between devices. Some typical CDMA telephony systems use combinations of “spreading codes” and “covering codes” to encode signals. These encoded digital signals are used to convey digital voice, data and/or other forms of digital communication. As used herein, spreading codes generally refer to “on-time” pseudorandom number, or pseudo-noise (“PN”), sequence codes. The particular timing alignment of a selected signal may be used in the extraction of the data from the assigned signal tracked within a receiver. Spreading codes and covering codes are known to those skilled in the art.
A spreading code encodes a data signal by applying the noise-like code sequence to the data at a rate faster than that of the data. For example, the spreading code is applied to the data such that there are multiple “chips” of the code for any given element of data. Such an application of the spreading code is commonly referred to as direct sequence spreading of the data. A short code is an example of a spreading code. Chips and their associated chip rates are known to those skilled in the art.
A covering code further encodes the signal to provide “channelization” of a signal. For example, each unique covering code as it is applied to a spread signal provides a unique communication channel for the spread signal. This channelization allows a signal to be divided into a number of individual communication channels that may be either shared or assigned to specific users. Covering codes typically include families of codes that are either orthogonal (e.g., Walsh codes) or substantially orthogonal (e.g., Quasi Orthogonal Function codes, or “QOF” codes). Such codes are known to those skilled in the art.
Interference may degrade signal detection, tracking and processing capabilities of a receiver by hindering the reception of a selected signal. In CDMA communications, for example, interference can be the result of receiving one or more unwanted signals simultaneously with a selected signal. These unwanted signals can be coded signals having properties that are similar to that of the selected signal. Because of the non-orthogonality of the codes and the corresponding energy of the signals, the coded signals often have a tendency to interfere with one another and disrupt reception of the selected signal. The lack of orthogonality in the codes used to encode the transmitted signals results in leakage between signals. Examples of such interference include “cross-channel” interference and “co-channel” interference.
Co-channel interference may include multipath interference from the same transmitter, wherein separate transmission paths follow unique routes that cause an interfering signal path and a selected signal path to differentially arrive at a receiver, thereby hindering reception of the selected path. Cross-channel interference may include interference caused by signal paths of other transmitters hindering the reception of the selected signal path. Such interference can interfere with data recovery as long as it is present in any substantial form. Interference is a continuing problem in communications, particularly in CDMA telephony.